Wheel shimmy is a condition in which the landing gear wheel or wheels oscillate from side to side along a straight line parallel to the direction of travel of the aircraft. Wheel shimmy can be caused by a number of conditions such as low torsional stiffness, excessive freeplay in the landing gear, wheel imbalance, or worn parts. Often, however, wheel shimmy occurs even in new aircraft due to resonance between the landing gear and the airframe of the aircraft. This resonance may not be discovered until after a new aircraft is first flown and the design of the landing gear and airframe is well established. Solving a shimmy problem after an aircraft is built presents challenges because it is not usually economical to redesign and replace the landing gear in order to solve the problem. Accordingly, several different concepts have been developed for reducing or eliminating wheel shimmy in existing landing gear.
For example, hydraulic shimmy dampers have been used to damp wheel shimmy. Typically, such shimmy dampers consist of a hydraulic shock absorber mounted between components of the landing gear to damp shimmy motions. The hydraulic shock absorber generally consists of a hollow tube filled with oil. A rod and piston move through the fluid to generate velocity-dependent, viscous-damping forces. Such designs require frequent maintenance, and temperature increases can reduce damping efficiency. Further, such shimmy dampers generally do not permit 360 degree rotation of the wheel or wheels and typically are difficult to adjust to provide different levels of damping.
Another type of shimmy damper that has been used in the past is a friction-type shimmy damper that mechanically engages a steering collar gear on a landing gear strut. For example, one design includes a belleville spring washer compressed against the steering collar to damp rotation thereof. While such a shimmy damper may be functionally adequate, it is not readily retrofittable to existing landing gear assemblies and installation and/or removal of the belleville spring washers typically requires the landing gear to be substantial disassembled, as many of these types of dampers require the spring washers to be telescoped over the landing gear strut.
Hydraulic damping (by addition of a restrictor) is also used as a feature of the many hydraulic steering actuation systems as a method to attenuate shimmy vibration. This method is popular as it adds the least additional weight and is generally effective. In cases where the hydraulic restrictor is not effective due to distance from the vibration source or flexibility of the structural load paths from the actuation system to the vibration, other methods may be required.